1. Field of the Invention
The present invention relates to a semiconductor laser module with a cooler system using a Peltier element, and more particularly, to the improvement of a metal substrate secured on the Peltier element.
2. Description of the Related Art
Semiconductor laser modules are currently used for optical fiber telecommunications particularly as signal light sources of cable televison (cATV) systems and excitation light sources of fiber amplifiers. Such semiconductor laser modules typically contain a Peltier element that operates as an electronic cooler in order to realize a high output power and stable operation of the module and a metal substrate is mounted onto the top of the Peltier element such that optical components such as a laser diode chip, a photodiode chip and/or a lens and electric components such as a thermistor, inductors and/or resistors are arranged on the metal substrate.
Such semiconductor laser modules are so configured as to cool the entire metal substrate and maintain the temperature of the laser diode chip to a constant level by detecting the temperature by means of the thermistor bonded near the laser diode chip and feeding back the detected temperature to the Peltier element to drive the latter.
Semiconductor laser modules using a Peltier element as a cooler are disclosed, inter alia, in JP-A62-117382 and JP-A-62-276892.
In a prior art semiconductor laser module, the metal substrate is made of a single metal having a large thermal conductivity. On the other hand, the ceramic panel is made of a ceramic having a small thermal expansion coefficient. In this case, in order to enhance the power output capability of the semiconductor laser module, the metal substrate is preferably made of a metal having a much larger thermal conductivity, to enhance the heat radiating effect. This will be explained later in detail.
In the above-described prior art semiconductor laser module, however, there is a large difference in thermal expansion coefficient between the metal substrate and the ceramic panel. Therefore, the metal substrate and the ceramic panel are subjected to large thermal stress. As a result, the Peltier element can become destroyed in a thermal environment test involving rapid temperature changes.